The g sounds like a j. Why is it written g?

The rule is old and almost invisible from inside the language. A g in front of e, i or y usually goes soft: gem, ginger, gyrate, general. In front of a, o or u it stays hard: gate, gone, gull. English applies this to Latin words as well as its own, which is why we say the g in selaginella as a j even though in Latin it was hard in every position. So the sound is French in origin, the letter is Latin, and the two have never been reconciled. The word to hold on to is Selago, where the g comes before an o and is unmistakably hard. Selaginella is that word with an ending on it, and the ending is what softened the g.

A g read as j, which English does before e, i and y.